"My certainty in victory is infinitely higher than on December 1, 2007," Chavez said.

Chavez is popular with many of the country's poor for health and education programmes, but blamed by a vocal opposition for rising crime, corruption and inflation.

His opponents say a victory for Chavez in the referendum would remove the last remaining check on his power.

Oscar Perez, a representative of the opposition, called Chavez's government "fascist" and "intolerant".

The opposition says Chavez called the vote, which takes place only three months after the opposition gained ground in regional and municipal elections, before the country feels the impact of falling oil prices - the main source of funding for the president's social programmes.

With a strong grasp on the private media, the opposition has focused its campaign on the importance of alternating power for democracy in their campaign, and accused Chavez of abusing state resources to fund a massive "Yes" campaign.

Voters undecided

About 100 international observers have been accredited to observe the referendum, but neither the Organisation of American States (OAS) nor the European Union have official observers in Venezuela.

Polls conducted at the end of the campaign show Chavez has a slight lead among those who have already decided.

Two pollsters on Saturday - Datanalisis, which works for the public and private sector, and Consultores 30.11, which works for the government - gave the "Yes" vote a lead of between five points and seven points.

Chavez has been widening his lead slightly since January, the pollsters said.

Both surveys conducted during the final week of the campaign also said more than 10 per cent of Venezuelans who planned to vote still had not made up their minds, making the outcome hard to predict.